The color vision of monkeys is to be investigated at the retinal, cortical and behavioral levels by a new method of light exchange that allows the activation of each cone type to be individually controlled. For example, it is possible to present the array of 'green' cones with a flashing or moving spot or bar, while the other cone types see only a uniform and unchanging field. At the retinal level this approach will be used, for each ganglion cell type, first to determine the receptive-field organization for each of the cone types separately, and then to determine what interactions exist between the signals from the different cone types. The color-discrimination properties of the ensemble of ganglion cells, so obtained, will be compared with the behaviorally determined color discrimination parameters for the same stimulus configuration. At the cortical level the cone inputs and cone interactions will be determined for the various cell types using stimulus patterns appropriate for the spatial receptive-field properties of these cells. Comparison with the retinal findings will provide basic information on the transformations in color space that take place between retina and visual cortex. Associated with the monkey color studies are three others that use the same exchange approach. Two are electrophysiological studies on cat retina and visual cortex, the third is a human psychophysical study. These studies, though self-contained in their own right, allow various aspects of the approach and apparatus to be tested prior to its application in acute monkey studies.